a) Field of the Invention
The invention is directed to processes and arrangements for the evaluation of laser Doppler signals, especially in laser Doppler anemometers (LDA) or vibrometers.
b) Description of the Related Art
At present, laser Doppler devices for the measurement of velocities and quantities derived therefrom such as length and acceleration are commercially available. They are distinguished from one another by optical construction, electronic control and evaluation for special measurement tasks. Typical known optical arrangements are LDA's based on the intersecting beam or crossbeam principle and the reference beam principle which are described thoroughly in Laser und Optoelektronik 20 (3), 1988, pages 73-78. The same reference source also contains an explanation of the distinction between the principles of control and evaluation which is drawn by the classification into the homodyne and heterodyne methods. In the case of the evaluation method, tracking methods have prevailed over frequency analysis methods and computer methods because of the possibility of real time demodulation of the Doppler signal which supplies an analog signal with velocity-proportional voltage at any time.
The technical problem basically consists in that the frequency range to be evaluated must always contain the entire width of the possible Doppler frequency (often several MHZ). On the other hand, the signal-to-noise ratio (SNR) is inversely proportional to the filter bandwidth. Therefore, the filter bandwidth must be kept as small as possible. In heterodyne laser Doppler devices, this can be realized in a simple manner based on the frequency offset by way of frequency tracker demodulation described, e.g., by DURST et al. (Theorie und Praxis der Laser-Doppler-Anemometrie [Theory and Practice of Laser Doppler Anemometry], G. Braun Verlag, Karlsruhe, 1987, pages 284-290). In this respect, an offset heterodyne tracking method, as it is called, proves especially advantageous (DURST, et al., p. 285).
A frequency tracker demodulation of this kind basically comprises a frequency mixer, a bandpass filter (IF filter), a frequency discriminator or phase discriminator, an integrator, and a voltage-controlled oscillator (VCO) which is fed back to the frequency mixer.
The Doppler signal is mixed with the output signal of the VCO and the resulting mixed signal with two sidebands is subjected to narrow-band filtration to the intermediate frequency. Changes in the Doppler frequency are compensated in the control loop outlined above by changing the VCO frequency. For this purpose, the integrator regulates the transient response and the stability of the control loop. In the heterodyne LDA, the frequency offset, which is very constant as a rule, is utilized for frequency tracking demodulation and a bandpass filter is used with the offset frequency as center frequency. The frequency of the VCO is variably adjustable by means of a discriminator to compensate for a Doppler shift. A distinction is made between frequency discriminators and phase discriminators.
In the case of frequency discriminators, every change in the Doppler frequency generates an error voltage proportional thereto because the intermediate frequency is changed precisely by the amount of the Doppler shift. With the error voltage at the output of the discriminator, the VCO readjusts its frequency until the intermediate frequency again coincides with the center frequency of the bandpass filter.
The disadvantage of the frequency discriminator consists in that the linear range of the voltage frequency characteristic is relatively narrowly defined, so that large changes in the Doppler frequency cannot be sufficiently compensated by the VCO.
Signal evaluating systems working with phase differences use phase discriminators. In this case, the output signal of the bandpass filter is initially transformed almost to a square-wave signal in an amplitude limiter. The output signal of the phase discriminator is the product of two square-wave signals. It corresponds to twice the center frequency (resonant frequency) when the intermediate frequency equals the center frequency of the bandpass filter. A deviation in the intermediate frequency causes the integrator to generate an error voltage which readjusts the VCO. The disadvantage of phase discriminators consists in the susceptibility to small phase variations caused by the finite transit time of scattering particles in the measurement volume. Therefore, in ensembles of scattering particles which traverse the measurement volume in different phase positions, a variable Doppler shift is recorded even at a constant velocity of the ensembles.
Further, a tracking method has also been developed for homodyne LDA's. Wilshurst and Rizzo (J. Physics E: Sci. Instruments 8, 47) disclose an autodyne evaluating method, as it is called, in which the reception signal carrying the Doppler frequency is mixed separately with two components of a VCO signal which are at a 90-degree phase offset. The high-frequency component is eliminated from the mix signal in each case by a low-pass before each mix signal is mixed with the other signal supplied through a differential element and the multiplication results are averaged by a summing amplifier. Apart from a lower signal-to-noise ratio compared with heterodyne trackers, the chief disadvantage in this tracking method is that it can not be used for small Doppler frequencies because it would then no longer be possible to filter out the interfering sideband without clipping the Doppler signal. In any case, accurate results are achieved only when the range of the anticipated Doppler shift is known beforehand and does not change suddenly and when no temporary dropouts of the Doppler signal are expected.
The above-mentioned signal dropouts in the case of discontinuous flows or particle movements represent the general flaw in the known methods of frequency tracking demodulation which decisively impairs the accuracy and stability of the Doppler frequency.
Further, solutions for phase-modulated LDA's are known in the art, wherein DE 195 37 647 A1 should be mentioned by way of example because it discloses a remarkable optical double modulation with two modulation frequencies with a whole-number frequency ratio, which modulation frequencies are coupled so as to be fixed with respect to phase and frequency, and evaluation of a sideband, wherein the anticipated maximum Doppler shift must be taken into account for the bandwidth of the bandpass which is fixed at a common multiple of the modulation frequencies. This always leads to deterioration of the signal-to-noise ratio.